It has become common that vehicles such as automobiles are mounted with an airbag device for absorbing an impact on an occupant during an emergency such as a collision by inflating and deploying an airbag in the cabin. Such airbag device generally has an airbag that is normally housed in a folded state in a structure of a vehicle and is inflated and deployed in an emergency, and an inflator that supplies gas to the airbag.
For example, a side airbag that is inflated and deployed between an occupant and a door portion is embedded in a back portion (seat back) of a seat on which the occupant is seated, and breaks through or pushes aside the seat and is expelled forward into the cabin during a vehicle collision. Methods for preventing the increase of internal pressure when normal inflation and deployment of an airbag is interfered with in such a side airbag device have been proposed (see, for example, Japanese Unexamined Patent Application Publication No. 2010-535121).
The airbag described in Japanese Unexamined Patent Application Publication No. 2010-535121 has a front chamber and a rear chamber, and is configured such that, when gas is supplied from an inflator disposed in the rear chamber, gas is supplied to the front chamber through an opening formed in the rear chamber, and the airbag is inflated and deployed. A venting system including a vent hole and a closing panel is disposed in the front chamber. In the initial stage of inflation and deployment, the vent hole is not covered, so that gas can be vented. In the final stage of inflation and deployment, the vent hole is covered by the closing panel to prevent the venting of gas.